Many methods and devices have been developed up to now for the determination of glucose in vitro or in vivo by optical means.
For instance, in PCT application WO No. 81/00622, there is disclosed an IR absorption method and apparatus for determining glucose in body fluids. According to this reference, absorption spectra of serum or urine, both transmissive or reflective, i.e. due to back-scattering effects, are measured at two distinct wavelengths .lambda.1 and .lambda.2, .lambda.2 being typical of the absorption of a substance to be measured (for instance glucose) and .lambda.1 being a wavelength at which the absorption is roughly independent of the concentration of the substance of interest. Then the pertinent measured data are derived from calculating the ratio of the absorption values at .lambda.1 and .lambda.2, the bands of interest being in the range of 940-950 cm.sup.-1 (10.64-10.54 .mu.m) and 1090-1095 cm.sup.-1 (9.17-9.13 .mu.m), respectively. In this reference, the source of irradiation is provided by a CO.sub.2 laser.
Swiss Pat. No. CH-612.271 discloses a non invasive method to determine biological substances in samples or through the skin using an attenuated total reflection (ATR) prism. This method relies on the passing of an infrared beam through a wave-guide (ATR prism) directly placed against a sample to be analyzed (for instance the lips or the tongue). The refractive index of the wave-guide being larger than that of the medium of the sample (optically thinner medium), the beam propagates therein following a totally reflected path, the only interaction thereof with said thinner medium (to be analyzed) being that of the "evanescent wave" component at the reflection site (see also Hormone & Metabolic Res./suppl. Ser. (1979), p. 30-35). When using predetermined infrared wavelengths typical of glucose absorption, the beam in the ATR prism is attenuated according to the glucose concentration in the optically thinner medium, this attenuation being ascertained and processed into glucose determination data. U.S. Pat. No. 3,958,560 discloses a non-invasive device for determining glucose in patient's eyes. Such device comprises a contact-lens shaped sensor device including an infrared source applied on one side of the cornea and a detector on the other side thereof. Thus, when a infrared radiation is applied to the area under measurement, light is transmitted through the cornea and the aqueous humor to the detector. The detected signal is transmitted to a remote receiver and a read-out device providing data on the concentration of glucose in the patient's eye as a function of the specific modifications undergone by the IR radiations when passing through the eye.
GB patent application No. 2,033,575 discloses a detector device for investigating substances in a patient's regions near to the bloodstream, namely CO.sub.2, oxygen or glucose. The key features of such detector comprise radiation directing means arranged to direct optical radiation into the patient's body, and receiver means for detecting attenuated optical radiations backscattered or reflected within the patient's body i.e. from a region below the skin surface. The detected signal is thereafter processed into useful analytical data. Optical radiations include UV as well as IR radiations.
Other references rather refer to the measurement or monitoring of other bioactive parameters and components such as blood flow, metabolic oxyhemoglobin and desoxyhemoglobin but, in reason of their close analogies with the aforementioned techniques, they are also worth reviewing here. Thus U.S. Pat. No. 3,638,640 discloses a method and an apparatus for measuring oxygen and other substances in blood and living tissues. The apparatus comprises radiation sources and detectors disposed on a patient's body, for instance about the ear to measure the intensity passing therethrough or on the forehead to measure the radiation reflected therefrom after passing through the blood and skin tissue. The radiations used belong to the red and very near infrared region, for instance wavelengths (.lambda.) of 660, 715 and 805 nm. The number of different wavelengths used simultaneously in the method is equal to the total of at least one measuring wavelength typical for each substance present in the area under investigation (including the substance(s) to be determined) plus one. By an appropriate electronic computation of the signals obtained after detection from absorption at these diverse wavelengths useful quantitative data on the concentrations of the substance to be measured are obtained irrespective of possible changes in measurement conditions such as displacement of the test appliance, changes in illumination intensity and geometry, changes in the amount of blood perfusing the tissue under investigation and the like.
GB patent application No. 2,075,668 describes a spectrophotometric apparatus for measuring and monitoring in-vivo and non-invasively the metabolism of body organs, e.g. changes in the oxido-reduction state of hemoglobin and cellular cytochrome as well as blood flow rates in various organs such as brain, heart, kidney and the like. The above objects are accomplished by optical techniques involving wavelengths in the 700-1300 nm range which have been shown to effectively penetrate the body tissues down to distances of several mm. Thus in FIG. 14 of this reference there is disclosed a device involving reflectance type measurements and comprising a light source for injecting light energy into a waveguide (optical fiber bundle) applied to the body and disposed in such way (slantwise relative to the skin) that the directionally emitted energy whch penetrates into the body through the skin is reflected or back scattered by the underlying tissue to be analyzed at some distance from the source; the partially absorbed energy then reaches a first detector placed also over the skin and somewhat distantly from the source. Another detector placed coaxially with the source picks up a back radiated reference signal, both the analytical and reference signals from the detectors being fed to a computing circuit, the output of which provides useful read-out data concerning the sought after analytical information.
Although the aforementioned techniques have a lot of merit some difficulties inherent thereto still exist. These difficulties are mainly related to the optical properties of the radiations used for making the measurements. Thus, radiation penetration into the skin depends on the action of absorbing chromophores and is wavelength-dependent, i.e. the light in the infrared range above 2.5 .mu.m is strongly absorbed by water and has very little penetration capability into living tissues containing glucose and, despite the highly specific absorption of the latter in this band, it is not readily usable to analyze body tissue volumes at depths exceeding a few microns or tens of microns. If exceptionally powerful sources (i.e. CO.sub.2 laser) are used, deeper penetration is obtained but at the risk of burning the tissues under examination. Conversely, using wavelengths below about 1 micron (1000 nm) has the drawback that, although penetration in this region is fairly good, strong absorbing chromophores still exist such as hemoglobin, bilirubin ad melanin and specific absorptions due to glucose are extremely weak which provides insufficient or borderline sensitivity and accuracy for practical use in the medical field. In addition, the ATR method which tries to circumvent the adverse consequences of the heat effect by using the total internal reflection technique enables to investigate depths of tissues not exceeding about 10 .mu.m which is insufficient to obtain reliable glucose determination information.